1. Field of the Invention
The present invention relates to a display control apparatus and a method therefor and, more particularly, to a display control apparatus and a method therefor, suitable for a display device having a display element having, e.g., a ferroelectric liquid crystal as an operation medium, for updating a display state so as to be able to hold the updated display state upon application of an electric field.
2. Related Background Art
A display device is generally used in an information processing system or the like, as an information display means having a function of visually expressing information. A CRT display device is most popular as such a display device. A CTR has a large volume because a considerably large length is required in the direction of thickness of the display screen. A compact display device cannot be obtained as a whole. Degrees of freedom in use of an information processing system having a CTR as a display device, i.e., degrees of freedom in installation locations and portability, are limited.
To compensate for these drawbacks, a liquid crystal display device (to be referred to as an LCD hereinafter) can be used. That is, according to the LCD, the display device can be made compact (particularly low profile) as a whole. A display (to be referred to as an FLCD or FLC display hereinafter) using a liquid crystal cell containing a ferroelectric liquid crystal (to be referred to as an FLC hereinafter) is included in such LCDs. One of the characteristic features of the FLCD lies in that the liquid crystal cell has a storage characteristic of a display state upon application of an electric field. More specifically, the FLCD has the liquid crystal cell having a sufficiently small thickness, FLC molecules are aligned in a first or second stable state in accordance with the direction of electric field application, and this aligned state is maintained after the electric field is removed. The FLCD has a storage characteristic by the bistable properties of the FLC molecules. The details of the FLC and FLCD are described in, e.g., Japanese Patent Application No. 62-76357 (U.S. patent application Ser. No. 174,980 filed on Mar. 29, 1988).
Unlike in the CTR and other liquid crystal display devices, the FLCD can be driven to obtain a sufficient time margin in a continuous refresh driving cycle of the display screen. In addition to the continuous refresh driving, partial rewrite driving for updating the display state of only a portion subjected to updating on the display screen can be performed.
When the FLCD is used as a display device in an information processing system by the same display control as that of the CRT, an FLC display updating time is relatively long. For example, the FLCD cannot often cope with a change in display information (e.g., a cursor, a character input, and scrolling), the display state of which must be immediately updated. Therefore, partial rewrite driving as one of the characteristic features of the FLCD is performed to increase an apparent display speed.
A conventional partial rewrite technique is to store all lines having the updated display contents and rewrite all the lines having the updated display contents by partial rewrite operations. For this reason, independent jobs are to be performed on a plurality of windows as in a multiwindow system, and the following problem is posed. For example, when character scrolling is being performed in a window different from a current window (to be referred to as an active window hereinafter), a partial rewrite operation is performed in both the active window and the window subjected to scrolling to result in a decrease in display content rewrite speed. Therefore, partial rewrite operations cannot be sufficiently enhanced.